Vacuum pumps are widely used in industry, materials handling and transport, research, medical and even agricultural applications. The degree of vacuum required determines which type of pump is most suitable.
Conventional vacuum pumps are typically driven by an electric motor or an internal combustion engine. The three most common types are the centrifugal blower, vane pumps and piston pumps.
Ejector type pumps are used to produce absolute air pressures of 60 mm Hg single stage and 10 mm double stage. More stages can be added, but at the very high vacuum needed for applications such as vacuum coating optical parts or for vacuum deposited thin films for microminiaturization other types of pumps, such as the rotary oil-sealed type or the diffusion pump are more suitable. A commercially-available hybrid combination, for example a steam jet/liquid ring pump may be the best choice for some applications.
Vacuum pumps are most effective when positioned as near as possible to their point of use in order to avoid long connector tubes which need to be evacuated each time vacuum is to be used. For example, where the vacuum is used by a robot for materials handling tasks, the pump is preferably positioned on the robot arm. However robot arm movements are slowed, or even prevented, if a heavy load is attached to such arm. Consequently, it is an advantage for vacuum pumps for such use to be compact and of light weight. Many conventional vacuum pumps having metal bodies and attached electric motors are quite unsuitable for such service.
Ejector pumps, formerly known as jet pumps, operate on the Bernoulli Principle by use of a nozzle discharging a high velocity gas stream across a suction chamber connected to the equipment to be evacuated. The gas to be evacuated is entrained by the high pressure gas and is carried into a venturi-shaped diffuser which converts the velocity energy of the high-pressure gas into pressure energy.
Any available pressured gas may be used as a power source, but in practice the gas used is either steam or air.
Ejector pumps have attractive advantages over other types in that they have no moving parts, and have low capital and maintenance costs. Disadvantages are that energy costs are higher; and although air is free, compressed air can be expensive relative to electricity. Noise may also be a problem, though an adequately-sized silencer fitted at the discharge port can reduce this to an acceptable level.
Vacuum pumps of any type, including the ejector type, may be connected in series for achievement of higher vacuum or in parallel for reaching the required vacuum more quickly. The two types of connection may be combined to produce a series-parallel pump array.
Multi-stage ejectors offer advantages in efficiency and in lower noise levels. Multi-stage ejectors produce more vacuum flow than compressed air consumption, as opposed to single stage pumps where more compressed air is consumed than is withdrawn in achieved vacuum evacuation. Noise levels of multi-stage devices are in the range of 55 to 75 dBA, usually not requiring a silencer, as compared to the typical 90 dBA to be expected from single stage ejectors making installation of a silencer mandatory.
In U.S. Pat. No. 4,696,624 the present inventor disclosed a method of producing an ejector device wherein a plurality of ejector units positioned in a common housing each has a suction chamber. The device is series-parallel type, and has flap valves allowing air passage from one chamber to the next.
A similar device is described and claimed by Lasto in a later U.S. Pat. No. 4,880,358.
The geometry of the optimum nozzle is mainly a function of the area of the motive gas nozzle and venturi throat, pressure of the motive gas, and suction and discharge pressures; further factors of secondary importance also have a bearing on the result. What is clear is that optimum desired geometry for a nozzle will change as the pump makes progress in evacuating a chamber. At start-up the ejector pump is expected to quickly remove large quantities of gas against little resistance, while towards the end of its activity the pump has to remove small quantities of gas against much higher resistance. For whichever situation the ejector nozzle is optimized, energy in the form of compressed motive gas is wasted at either the beginning of pumping or towards the end, because the nozzle form and dimensions cannot suit the changing conditions of operation.
This problem is recognized by Tell, who proposes in U.S. Pat. No. 5,205,717 a method of achieving, with at least two compressed air operated ejectors, a desired sub-pressure in the shortest possible time and with the least use of energy. The ejectors are connected to work one at a time in response to whichever of them is supplied with compressed air. Compressed air supply is controlled in response to the sub-pressure in a collection chamber common to all ejectors.
An ejector array for the method includes at least two nozzles each having an optimum efficiency at a different value of supplied compressed air. A sensor measures sub-pressure in the common chamber and directs compressed air to one ejector at a time in response to measured sub-pressure. In operation the ejector operating best for evacuating large volumes first receives compressed air, and the nozzle operating best when evacuation pressure is low receives compressed air last.
A commercially available range of ejector-type vacuum pumps is marketed by PIAB. Lowest vacuum claimed to be achievable is between 5 and 100 millibar, depending on the model chosen.
A disadvantage of prior art ejector pumps is that efficiency is impaired by the transfer of air in intermediate chambers, that is between stages, between two parallel air streams, one of which is optimized for large volume low resistance pumping while the second stream is intended for low volume high resistance pumping. Such undesirable air transfer is made possible by the use of a common intermediate chamber for the two air streams.
It is therefore one of the objects of the present invention to obviate the disadvantages of prior art ejector pumps and to provide a pump which operates more efficiently both at the start and towards the end of vacuum draw-down.